1. Field of the Invention
This patent specification relates to pressure transducers. More particularly, this patent specification relates to resonator-based pressure transducers having non-circular cross sections and to improved methods for manufacturing the same.
2. Background
Piezoelectric pressure and temperature sensors typically have a crystal resonator located inside a housing with electrodes. Environmental pressure and temperature are transmitted to the resonator, via the housing, and changes in the resonator are sensed and used to interpret the pressure and/or temperature. U.S. Pat. No. 3,617,780, incorporated herein by reference, describes one example of a piezoelectric pressure transducer. In conventional devices, known as single-mode transducers that utilize single-mode oscillation, the resonator is affected by both temperature and pressure such that some devices may not be suitable for use in environments where both temperature and pressure vary.
One approach that is utilized to minimize fluctuations in pressure measurements is to use resonators with dual-mode oscillation. U.S. Pat. Nos. 4,419,600, 4,547,691 and 5,394,345, all incorporated by reference herein, disclose examples of such pressure transducers. However, transducer geometry for such resonators tends to be relatively complex, and the transducer tends to be larger due to the manufacturing process. Under certain conditions, such as in oil or gas wells, stress on the transducer may cause material twinning or micro-cracks that might damage the pressure transducer.
FIGS. 1A and 1B depict examples of known pressure transducers having circular cylindrical outer shapes. In particular, FIG. 1A depicts an example of a classic pressure transducer in which resonator 110 has two vibrational modes at different frequencies. Housing 112 has a circular cylindrical outer shape. FIG. 1B depicts an example of pressure transducer having an axisymmetric housing that includes two end caps 122 and 124. For further details of such transducers, see U.S. Patent Publication No. 2009/0009036, which is incorporated by reference herein.
Conventional manufacturing processes for pressure transducers such as shown in FIGS. 1A and 1B using mechanical lapping and/or grinding of individual components and assembly of a single transducer. For example, the transducer shown in FIG. 1B is made of two end caps 122 and 124, and a resonator 120. Each of these pieces are mechanically shaped using, for example, chemical mechanical polishing, lapping and/or grinding. After shaping, the pressure transducer is assembled using, for example, a bonding process.
Such conventional manufacturing processes are relatively expensive due to the mechanical shaping of the individual components and assembly of each pressure transducer unit. Additionally conventional designs are relatively large in size, which leads to more material being required which further increases the cost. For example, the classic design shown in FIG. 1A is typically about 22 mm in outer diameter. The design shown in FIG. 1B is substantially smaller, typically about 15 mm in outer diameter. However if further miniaturization can be accomplished, larger cost savings could be realized.
U.S. Pat. No. 4,554,717 discusses a method of making square-shaped quartz crystal resonators using chemical polishing and photolithographic techniques. There is no discussion of using the method for manufacturing pressure transducers.
Thus, there is a need for a more cost effective design for resonator-based pressure transducers having improved manufacturability.